The invention relates to an exhaust valve mechanism for an internal combustion engine.
When a heavy duty Diesel engine is operated under low load and/or idling, the exhaust temperature is low or even far too low for an Exhaust After Treatment System (EATS) comprising a catalyst to function properly. At low engine speeds, it is typically necessary to provide an auxiliary burner in the post-turbine exhaust. Such a solution involves the use of additional fuel.
One way to modify the thermodynamic process of the engine during low load operation in order to increase the exhaust temperature is to shorten the duration of the expansion stroke. A shortened expansion stroke will result in less torque to crankshaft and energy remaining in the exhaust gas expressed as higher temperature. This elevated exhaust temperature is very beneficial for the function of EATS. The reduced torque delivered to the crankshaft has to be compensated by increased fuelling, which further on will increase exhaust temperature.
An engine valve mechanism is needed which is able to switch between a normal mode of operation and a low load mode of operation to open the exhaust valves against the cylinder pressure a short duration (10-60° crank angle) after low load combustion. For a heavy duty truck engine, the valve mechanism also in many cases should be able to operate in brake mode to decompress the cylinder gas pressure just before TDC in the compression stroke.
Several engine valve systems of the so called lost motion type are known in the field that are able to perform several different valve motion events. However, these systems rely on electronic control by crank angle resolution of a hydraulic link in the valve train mechanism between camshaft and valve. Thus, these systems require an electric actuator for each cylinder. The complexity of these valve control systems makes them expensive and functionally more vulnerable than a mechanical system.
An exhaust valve mechanism for an internal combustion engine is disclosed in U.S. Pat. No. 6,983,725 B2. Each exhaust rocker arm is provided with two hydraulic pistons with a hydraulic communication. The pistons are engaged during engine brake operation. One piston, the slave piston, is situated between valve yoke and exhaust rocker arm. When the exhaust brake system is activated, the slave piston and the hydraulic cushion will adjust the valve lash to zero between exhaust rocker (exhaust cam lobe) and exhaust valves. The other piston, the master piston, is pushed by hydraulic pressure against its stop ring at upper position when the system is engaged. A brake cam lobe engages a brake rocker arm and the movements of the brake rocker are transmitted via a hydraulic link between the master piston and the slave piston to the exhaust rocker arm to perform the engine brake events.
It is desirable to provide a hydraulic valve actuation system with more than two operating modes without the need for electronic control means.
According to an aspect of the present invention, an exhaust valve mechanism is characterized in that a second resilient member acts on the hydraulic link with a spring rate lower than said first resilient member to allow the activation of an extra valve event.